Auto update

Update miniflux by fetching a zip file of miniflux source code. 

The update system will try (in this order) : 

* save the current files (freeze), 
* download the zip file, 
* extract the zip file in a temporary directory 
* finally copy the new files. 

If an error occurs durring the last step, the files saved at the setp 1 will be restored. 

This update system **do not** :

* check the effective version of miniflux
* replace only the updated/modified files

**zip file format** :
the update function will expected a zip file formatted like that

	master.zip
		|-miniflux-master (the name doesn't matter, it will take the first folder)
		|- assets
		|- cronjob.php
		| ...

The default zip file is https://github.com/fguillot/miniflux/archive/master.zip .
This url can be customized with the new option `update_url`.